1. Field of the Invention
This invention relates in general to digital filtering, and more particularly to combined parallel adaptive equalizer/echo canceller that minimizes memory requirements and circuit complexity.
2. Description of Related Art
xe2x80x9cCommunicationxe2x80x9d is the exchange of thoughts, opinions, ideas, and information. It is the means to socialize, do business, educate, and entertain. Communication can take many forms, such as spoken words, written letters, or symbols. Although face to face communication is often desirable, it is often not possible due to geographical distance, time constraints, and an ever-increasing need for a high volume of information in today""s society. It is for this reason that information, or data, is sent over communications xe2x80x9cchannels,xe2x80x9d via xe2x80x9csignals.xe2x80x9d
A communications channel is a single path for transmitting an electrical signal, such as a twisted wire-pair cable, or a fiber optic line. A signal is a physical representation of data, such as the electrical pulses which are used to correspond to digital logic levels.
Signals are sent, or transmitted, in a tremendous variety of forms. For example, signals are used to send voice information over a telephone line; modems use signals to transmit data between computers; signals are constantly sent between the CPU and disk storage device in a personal computer; and signals representing images and sound are transmitted from a television camera on-site, to the television in a viewer""s living room that could be thousands of miles away.
Signal distortion or degradation is a significant problem in the field of communications. Any real communications channel has transmission deficiencies, including various kinds of noise and interference, which distort the signal. For example, static noise (caused by natural electric disturbances in the atmosphere) and thermal noise (caused by the random motion of electrons in the channel) are present to some extent in any communications channel. Intersymbol interference (degradation caused by imperfect channels) can also be a major problem. In short, there are many reasons why a signal that is sent may be unrecognizable when it is received.
Thus, transmission deficiencies must be corrected so that the signal received is the same as the one that was sent, and valuable information is not lost. This correction can be accomplished by the signal receiver, through a process known as equalization.
Equalization is the process of correcting a channel for its transmission deficiencies, by introducing networks which compensate for attenuation and time delay problems in the signal. A properly equalized communications channel will significantly increase the likelihood of obtaining an accurate signal (i.e., the signal that was sent) at the receiving end of a communications network. An equalizer is a device used to accomplish equalization.
In addition, impedance mismatches in the transmission media often cause signal echoes. Echo cancellation is the process of eliminating such echoes from the signal path. To cancel the echo signal, a basic operation is implemented: subtraction. Overall, an estimate of the echo signal is generated by adaptive compensation circuit and then subtracted from the echo signal itself. The compensation circuit is fed by both the original signal to be transmitted and the residual signal that results after the echo cancellation takes place. The original signal is used to create the echo signal estimate and the residual signal is used for the adaptation process within the compensation circuit to improve the quality of the estimate echo signal. The adaptive compensation circuit and the subtraction circuit, the circuit that takes the estimate and subtracts it from the received signal, form the echo canceller.
A filter is generally used in equalizer/echo canceller circuits. A filter may have a means of monitoring its own frequency response characteristics and a means of varying its own parameters by closed loop action, in order to attain optimal equalization or echo cancellation. Such a self-adjusting filter is called an adaptive filter, and it can be used in a channel receiver. The parameters of an adaptive filter are typically adjusted by sampling the filter output at a predetermined rate, and sending this sampled output to some filter control means, which adjusts filter parameters accordingly via closed loop feedback.
Commonly, a feed forward equalizer or echo canceller has weighted summed delayed versions of an input signal, which are used to derive an error signal. Further, known Least Means Square (LMS) algorithms have generally been implemented to negate the effects of channel-induced intersymbol interference and to promote efficient echo estimation. To accomplish this, periodically running cross-correlations that are weighted by a feedback weighing factor are loaded into a finite impulse response (FIR) filter. The cross-correlation used for a given filter coefficient corresponds to the symbol data sample position of a delay register and a relative delay of the error corresponding to that sample. In doing this, a tap weight vector update is produced.
However, the standard transversal LMS implementation has several disadvantages or limitations. For long filter applications, the summation tree formed with the summation blocks present latency problems. Such latency problems are especially problematic in very high-speed applications such as Gigabit Ethernet or Fast Ethernet. Further, an adaptive filter must have a sufficient number of taps to provide the requisite sampling of the signal. However, the performance of the transversal equalizer is improved by increasing the sampling rate to a value at least greater than twice the bandwidth of the received signal. Thus, memory requirements increase with increased bandwidth.
It can be seen that there is a need for a combined adaptive equalizer/echo canceller that provides low latency for high-speed applications.
It can also be seen that there is a need for a combined adaptive equalizer/echo canceller that minimizes the memory requirements for implementing the filtering/echo cancellation functions.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses digital filter.
The present invention solves the above-described problems by providing a combined parallel adaptive equalizer/echo canceller that minimizes memory requirements and circuit complexity.
A system in accordance with the principles of the present invention includes at least one transverse finite impulse response filter receiving a plurality of taps from a data input signal that are processed using tap coefficients to produce a plurality of tap outputs, an additive pipeline for receiving the tap outputs and processing the tap outputs through the pipeline to produce an output signal and a coefficient processor for calculating updated tap coefficients, the updated tap coefficients calculated using an error signal and delayed versions of the input signal.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that a most recent tap output is received by the pipeline one cycle from the output signal.
Another aspect of the present invention is that each filter receives an input signal and provides n tap signals to corresponding n multipliers, wherein the nth multiplier provides an nth tap output signal resulting from the product of the nth tap signal and an nth tap coefficient.
Another aspect of the present invention is that the pipeline further includes n adders having inputs for receiving the n tap output signals and an output for providing a sum signal representing the sum of the n received tap output signals and n-1 registers having an input and an output, the input of each register being coupled to an output of an adder, and the output of each register being passed to an input of a next one of the adders in a pipeline manner according to a next cycle.
Yet another aspect of the present invention is that the tap outputs are received by the pipeline such that each nth adder receives all nth tap output signals.
Another aspect of the present invention is that the error signal comprises a product of a calculated error and a negative adaptation factor.
Another aspect of the present invention is that the delay is equal to a sum of one cycle and a length of the at least one transverse finite impulse response filters.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.